Suction filters known in the art generally have a discharge opening in the circumferential wall of the filter housing, located above the filter plate. At the discharge opening, after a cake of the particulate matter is formed by filtering, the filter cake is generally cut and pushed to the outside by cutting arms and pushed out through the discharge opening. In such case, the discharge opening should be at least as high as the filter cake height, and relatively large, which may pose problems when the filter housing has a relatively small diameter. Further, the filter housing may be deformed during assembly, so that it is no longer exactly round. Filter cake residues can remain in the container edges in such a case, as the wiping arms will not reach to the edge throughout their sweep when an out-of-round housing is involved. Also, if the container bottom must be joined in a complicated way with the housing, as is generally the case, residues can become stuck between the bottom cover and the housing.
To solve such problems, an outlet opening and line are generally centrally located under the filter plate, i.e., in the longitudinal axis of the filter. In W. German patent No. DE-PS 12 98 081 a suction filter is taught in which the filter cake is carried from the filter plate to the outlet opening by a screw conveyor. However, since the conveyor is open over the discharge line, it must be sealed fluid tight at that point. Since highly corrosive and toxic agents are frequently handled by the filter, the use of expensive materials in the fabrication of the filter construction is mandated to achieve the seal. Also, valuable filtering space for the formation of the filter cake is lost. Furthermore, the screw conveyor is subject to wear and down-time for maintenance, which can become prohibitively expensive.
In the DE-OS 31 36 773, a filtering and drying apparatus is described which has a sealing structure in the outlet line. However, necessary space for the filter cake on the filter plate can be lost to the seal structure. Alternatively, the discharge of the filter cake through the central outlet opening can require a high pressure differential in order to initiate the movement of the cake, which lies as a solid mass over the discharge opening. With this apparatus the discharge of the filter cake is also relatively expensive and unsatisfactory.